Clad system for brazing to alumina

ABSTRACT

Alumina is bonded to a metal by means of an active brazing filler alloy. When the metal to which the alumina is bonded is a high blushing metal, that is, a metal on which the active brazing filler alloy readily flows on the surface, a thin layer of a low blushing metal is disposed therebetween.

This invention is concerned with the brazing of alumina ceramic to ametal or ceramic part. Such a braze requires an active filler metal, asis known, and as is disclosed in, for example, U.S. Pat. Nos. 4,643,875,4,623,513 and 4,448,605. It is also known that a layer of a cushioningor ductile metal should be provided between the alumina and the part towhich it is to be brazed, if there is a mismatch of coefficients ofthermal expansion, in order to absorb stresses. Such a cushioning layeris disclosed in U.S. Pat. No. 4,562,121.

It can occur that the cushioning metal used for stress relief is one onwhich the active brazing filler metal blushes, that is to say, theactive brazing filler metal flows on the surface of the cushioningmetal. When such blushing occurs, it leaves very little of the activeelement, usually titanium, in the filler metal to wet the ceramic whenthe ceramic material reaches the brazing temperature. One way toalleviate this situation is to use a thicker layer of the active brazingfiller metal. However, since an expensive metal, usually silver, isoften a major constituent of the active brazing filler alloy, thissolution can be undesirably expensive.

This invention is concerned with another solution to the blushingproblem. In this invention, the cushioning metal layer is covered with athin layer of a low blushing metal on which the active brazing fillermetal does not readily flow.

This invention can also apply to a situation where, instead of acushioning metal layer, the alumina ceramic is to be joined to a metalhaving about the same coefficient of expansion as the alumina, forexample Alloy 42 (42% Ni, 58% Fe); alumina is 8×10⁻⁶ per °C., whileAlloy 42 is 10×10.sup.⁻⁶ per °C.

In the case where the active brazing filler alloy comprised silver,copper and titanium or silver, copper, titanium and indium, I have founda satisfactory low blushing metal to be 410 or 430 stainless steel.

EXAMPLE 1

A 0.40" 410 Stainless is placed on both faces of 0.420" thickness Alloy42 and the combination rolled down to 50 mil overall thickness. Thecomposite consists of a 4 mil 410 Stainless Steel skin, 42 mil Alloy 42and a 4 mil 410 Stainless Steel skin.

The 410 Stainless Steel-alloy 42 composite is placed between1.2"×1.2"×0.100" AL-995 alumina substrates with 3 milsilver-copper-titanium alloy (sold under trade name of Cusil ABA andmanufactured by WESGO, Division of GTE Products Corp.) on both sides ofthe composite. The assembly with 100 gram steel load is placed in avacuum furnace and brazed at 840° C. under 10⁻⁵ Torr vacuum. Anexcellent bonded system is produced.

EXAMPLE 2

Two AL-995 alumina ceramic tubes, 1" outside diameter with 0.75" insidediameter are joined by placing a 1" diameter circular disc by 10 milthickness 410 Stainless Steel clad Alloy 42 between the tubes. Twopreforms of Incusil ABA with a dimension of 1" O.D. by 0.75" I.D. by 2mils thickness is placed on either side of the clad metal. The assemblyis brazed at 750° C. under 10⁻⁵ Torr vacuum. The joint showed excellentbraze and He gas mass spectograph leak checked to better than 10⁻⁹cc/sec. leak rate. Incusil ABA is WESGO's trade name for asilver-copper-indium-titanium alloy.

EXAMPLE 3

Same test as Example 2 in all details except the clad metal was replacedby 10 mil 42 metal preform. The brazed sample showed that the fillermetal flowed over the Alloy 42 leaving very little fillet. The jointshowed a leak rate of about 10⁻⁶ cc/sec. which is unacceptable.

EXAMPLE 4

Same test as Example 3, except a 4 mil thickness Incusil ABA preform wasused. The result showed that 4 mil thickness preform resulted with ahermetic joint measuring better than 10⁻⁹ cc/sec. He leak rate. However,the brazed sample showed excessive blushing.

In summary, the invention comprises a clad system of a relatively thickmetal layer, on which an active brazing filler metal blushes, coveredwith a thinner layer of a metal on which the active brazing filler metalblushes little or none. The relatively thick metal layer can be acushioning metal, such as molybdenum or Kovar (Kovar is a trade name ofWestinghouse Co. for an alloy of FeNiCo), or a metal selected for havinga coefficient of thermal expansion close to that of alumina, such asAlloy 42. The low blushing metal can be 410 or 430 Stainless Steel.

I claim:
 1. The method of bonding alumina to a cushioning metal which ishigh blushing to an active brazing filler metal comprising the steps of:roll bonding a thin layer of a lower blushing metal to a thicker layerof the cushioning metal and rolling the combination to a predeterminedthickness; disposing a thin layer of an active brazing filler metalbetween the lower blushing metal and the alumina; and melting the activebrazing filler metal to bond the alumina to the cushioning metal.
 2. Themethod of claim 1 wherein the active brazing filler metal comprises asilver-copper-titanium alloy or a silver-copper-titanium-indium alloy.3. The method of bonding alumina to a metal having a closely matchingcoefficient of thermal expansion and which is high blushing to an activebrazing filler metal comprising the steps of: roll bonding a thin layerof a lower blushing metal to a thickner layer of the matchingcoefficient metal and rolling the combination to a predeterminedthickness; disposing a thin layer of an active brazing filler metalbetween the lower blushing metal and the alumina, and melting the activebrazing filler metal to bond the alumina to the matching coefficientmetal.
 4. The method of claim 3 wherein the active brazing filler metalcomprises a silver-copper-titanium alloy or asilver-copper-titanium-indium alloy.